This invention relates to improvements in log charger systems for veneer lathes. More particularly the invention relates to improvements in such charger systems having automated scanning equipment for sensing the shape of a log and positioning the log for optimum production of veneer.
In the design of veneer production equipment, the primary objectives are to maximize the yield of usable veneer from the irregularly-shaped logs from which the veneer is peeled, and to maximize the production rate of the veneer. In order to attain these objectives great effort has been expended in the development of sophisticated, automated log scanning equipment, primarily of the electro-optical type, for sensing the shape of each log and rapidly determining its longitudinal axis for optimum veneer production. Examples of such scanning systems used or usable for this purpose are contained in the following U.S. Pat. Nos. 3,736,968; 3,746,065; 3,787,700; 3,852,579; 3,890,509; 3,902,539; 3,992,615; 4,197,888; and 4,221,973. Electro-optical scanners constructed in accordance with the foregoing technology, and particularly those which rotate the log during the scanning process, are extremely accurate and have the capability of determining the location of the log axis for optimum veneer production to within a few thousandths of an inch.
While such a high degree of accuracy in determining the optimum peeling axis should theoretically maximize the yield of veneer from each log, the results obtainable in practice have unfortunately fallen short of this goal because the mechanical log manipulators of veneer lathe chargers are incapable of duplicating the scanner's degree of accuracy. Thus, although the scanning system may identify the location of the optimum peeling axis of a log to within a few thousandths of an inch, the mechanical log manipulators responsible for aligning such axis with the rotational axis of the lathe actually allow a much wider margin for error than that tolerated by the scanning system. Because of this discrepancy in tolerances between the electro-optical and mechanical portions of veneer lathe chargers, substantial mispositioning of the logs and less than optimum yields persist despite the provision of the highly accurate scanning systems.
The progress of log manipulating mechanisms, as opposed to scanning systems, in veneer lathe chargers is exemplified by U.S. Pat. Nos. 3,037,538, 3,664,395, 3,746,065, 3,752,201, 4,197,888 and 4,246,940. In general, all of such chargers attempt to hold the log at either a prepositioning or a scanning station to determine its optimum peeling axis, adjust the position of the log such that the optimum axis is aligned with a reference axis, and transfer the log to the veneer lathe such that the optimum axis is aligned with the rotational axis of the lathe. In these few mechanical steps, however, there are many opportunities for log positioning errors. Relatively large errors can occur, for example, if at any point from the initiation of prepositioning or scanning to the securing of the log in the lathe, the log is supported by engagement with its curved surfaces at points intermediate its ends, rather than by end engagement. Surface engagement allows variation of log position due to surface irregularities or differences in resistance to compressive forces along the surface area, either of which can shift the position of the log transverse to its optimum axis and thereby cause significant error in the final alignment of the optimum axis relative to the veneer lathe. Such error can occur even in those chargers which primarily utilize end engagement of the logs if, at only a single point in the manipulation of the log after the initiation of scanning, surface engagement rather than end engagement is utilized. For example, in U.S. Pat. No. 4,197,888 surface clamp arms are required to hold the log in position between release of the log by the end-engaging rotary scanning spindles and end engagement of the log by the transfer arms which move the log into the lathe. In less modern chargers such as those shown in U.S. Pat. Nos. 3,037,538, 3,664,395, 3,746,065 and 3,752,201 the error problem is further compounded by the fact that rotary scanning is not used and therefore the optimum axis is determined from insufficient information regarding the log's profile.
From the foregoing it will be appreciated that sufficient accuracy in log positioning can be obtained only by utilizing end-engaging log manipulation techniques exclusively after the initiation of the scanning step. That is, the log should be end-engaged exclusively during scanning, during the transfer of the log from the scanner to the transfer mechanism, during movement of the log by the transfer mechanism from the scanner to the lathe, and during transfer of the log from the transfer mechanism to the lathe. End engagement at all of these points in the log manipulation process, as shown for example in the charger of U.S. Pat. No. 4,246,940, while conducive to accurate positioning does however raise another problem of error which must also be overcome. Reliable end engagement of logs normally requires penetration of the opposing ends of the logs by spikes or similar devices. In such a charger employing end engagement exclusively the rotary scanner spindles, for example, conventionally penetrate the ends of the log in the same locations eventually penetrated by the veneer lathe spindles. Accordingly, the cavities created by the scanner spindles tend to deflect or misguide the ultimate penetration by the lathe spindles, thereby tending to force the log out of proper alignment as it is engaged by the lathe spindles. Similar problems can occur in the successive engagement of a log by any two end-engaging manipulators.
Mechanical error in log positioning also results from wear of log manipulators. Manipulator positioning devices, such as fluid power ram assemblies, usually function also as position sensors for the manipulators so as to feed position information back to the servo valves which control the positioning devices. The connection between a respective positioning device and a log manipulator often becomes worn because of the large actuating forces transmitted through the connection. When this happens, the accuracy of the positioning device's second function as a true position sensor becomes impaired resulting in log positioning errors.